This invention relates generally to keyboard assemblies and more particularly to such assemblies having significant key travel and momentary type switches.
Keyboards of the type having snap acting, electrically conducting discs arranged to electrically connect two spaced conductors when a disc is actuated from one dished configuration to a second, oppositely dished configuration, are well known and in wide use. The spaced conductors may include staple like elements mounted on a printed circuit board with the discs maintained in selected locations relative to the staples by a disc retainer member, essentially a sheet of electrically insulative material having a plurality of disc receiving apertures formed therein. A keyboard of this, type is shown and described in U.S. Pat. No. 3,858,202.
In U.S. Pat. No. 4,005,293 the discs are formed in strips, the discs of a strip being electrically and physically connected to each other. The strip is attached to a wire conductor embedded in a plastic substrate with the discs adapted upon actuation to move into engagement with other wire conductors embedded in the substrate and extending parallel to the first referenced conductor.
Keyboards of this general type are very effective and reliable and provide a desirable tactile feedback characteristic, that is, when the discs snap from one configuration to an opposite configuration upon being depressed by a user the snap movement is felt by the user providing a positive indication of switch actuation. However, due to the nature of the switching mechanism the ultimate cycle life of the keyboard is not as high as is desirable for certain applications. That is, the disc is formed as a dish to set up inherent stresses to provide the snap motion thereby decreasing the ultimate life of the disc. Further, the cost of the keyboard, per switching station, is higher than desirable.
Another type of keyboard having very high cycle life has become conventional which comprises first and second sheets of electrically insulative material, such as polyester, on each of which are deposited electrical conductors and which are separated from one another by a spacer sheet of electrically insulative material. The spacer sheet is provided with apertures aligned with selected portions of the deposited electrical conductors so that depression of the top sheet at one of the selected locations will cause the selected portions on the first and second sheets to come into contact with each other. While this type of keyboard is very inexpensive as well as having a long life, it suffers from having very little travel, that is, essentially the thickness of the spacer, perhaps 0.004 or 0.005 inch. Further, there is no tactile feel for providing assurance that a keyswitch has been actuated.
To overcome this limitation bubbles have been formed in one of the polyester sheets to increase the travel before switch actuation and to provide some form of tactile feedback. The use of these bubbles however markedly decreases the cycle life of the switch, for example from millions of cycles to two or three hundred thousand cycles.